The invention relates to a material for aging-resistant ceramic vaporizers.
The most widely used process for coating flexible substrates with metals, in particular with aluminum, is high-vacuum tape coating. In a metalization installation, the substrate to be coated is passed over a cooled roller and, as it does so, is exposed to the aluminum vapor, which is deposited on the substrate surface as a thin metal layer.
To generate the constant vapor stream required, ceramic vaporizers are heated to about 1450xc2x0 C. by direct passage of electric current. Aluminum wire is continuously fed in, is liquefied on the ceramic surface and is vaporized at a vacuum of approximately 10xe2x88x924 mbar.
In metalization installations, a series of vaporizers are arranged in such a way that a uniformly thick aluminum layer is deposited over the entire width of the tape. The vaporizers are held by cooled copper clamping jaws, both end-side and lateral clamping being possible.
The width and length of the vaporizer are selected according to the vaporization rate required (g Al minxe2x88x921). The usual cross section of vaporizers is rectangular. As well as vaporizers with a rectangular cross section, a range of other cross sections are also known. Details of these are to be found, for example, in U.S. Pat. No. 4,089,643 or DE 19823908 (corresponds to the U.S. application bearing Ser. No. 09/315,320).
The resistance R of the vaporizer in the electric circuit can be calculated from the geometry of the vaporizer and the electrical properties of the ceramic material (characterized by the resistivity Rspec) in accordance with equation (1).   "AutoLeftMatch"                              R          =                                    R              spec                        xc3x97            L            ⁢                          /                        ⁢            A            ⁢                          xe2x80x83                        ⁢            L            ⁢                          :                        ⁢                          xe2x80x83                        ⁢            Length            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            vaporizer                                                        A          ⁢                      :                    ⁢                      xe2x80x83                    ⁢          conductive          ⁢                      xe2x80x83                    ⁢          cross          ⁢                      xe2x80x83                    ⁢          section                    
In the electric circuit, the vaporizer behaves as a metallic resistor, i.e. its resistance increases as the temperture rises. Therefore, it is additionally necessary to know the ratio between resistance at operating temperature (RHT) and the resistance at room temperature (RRT) in order to accurately characterize the electrical properties. For ceramic vaporizers, the ratio RHT/RRt is between 2.3 and 6.0.
A distinction is drawn between 2-component vaporizers (predominantly comprising TiB2 as conductive phase and BN as insulator) and 3-component vaporizers (predominantly comprising TiB2 as conductive phase and a mixture of BN and AiN as insulator), according to the number of principal components in the composition of the vaporizers in the 3-component vaporizer, AIN replaces some of the BN for economic reasons, since AiN is considerably less expensive. Therefore, in general the 3-component vaporizer contains 15-20% AlN. Such compositions are described, inter alia, in patents (e.g. U.S. Pat. No. 3,915,900, column 1, lines 10 and 11 and U.S. Pat. No. 4,089,643, column 2, line 5).
Since 2-component vaporizer materials have a lower high-temperature bending strength than 3-component vaporizer materials, they generally also contain 0.1-5% by weight CaO.
After production, 2-component vaporizers are generally packaged in bags made from metalized film, in order to protect the vaporizers from taking up moisture during transport or storage prior to use. This is because it has been found that 2-component vaporizer material is damaged in a characteristic way by the uptake of moisture. The damage becomes apparent from a strongly rising resistance during heating of the vaporizer, which takes place by the direct passage of current in metalization installations. In practice, this means that a vaporizer made from the damaged material cannot be heated in metalization installations, since its resistance is too high for conventional power supplies (10-15 volts secondary).
Despite this drawback, 2-component vaporizers are frequently used, since they can be operated at higher temperatures and can therefore reach vaporisation rates which are up to approximately +50% higher than 3-component vaporizers. Furthermore, they have a higher service life than 3-component vaporizers.
The object of the invention is to provide a ceramic material which is suitable for the production of 2-component vaporizers and is better able to withstand atmospheric humidity and is therefore more resistant to aging than known materials for 2-component vaporizers.
The object is achieved by a ceramic material containing 45 to 55% by weight TiB2 and 40.5 to 54.7% by weight BN and 0.1-1.5% by weight Ca in the form of a compound selected from the group consisting of the calcium oxides and calcium borates and mixtures thereof, which material contains from 0.2 to 3% by weight of a compound selected from the group consisting of the oxides, carbides and nitrides of the elements Al, Si, Zr, Ti and mixtures thereof.
The present invention provides a process for producing a ceramic material, said ceramic material containing 45 to 55% by weight TiB2 and 40.5 to 54.7% by weight BN; and 0.1-1.5% by weight of CaO; which material contains from 0.2 to 3% by weight of a compound selected from the group consisting of the oxides, carbides and nitrides of the elements Al, Si, Zr, Ti and mixtures thereof; comprising the steps of introducing from 0.2 to 3% by weight of said compound selected from the group consisting of the oxides, carbides and nitrides of the elements Al, Si, Zr, Ti and mixtures thereof, as a powder, in finely distributed form into a TiB2/BN/CaO mixture to form a homogeneous powder mixture for 2-component vaporizers, and hot pressing said homogenous powder mixture in a graphite mold using ram pressure, at temperatures of at least 1800xc2x0 C.
The present invention also provides a ceramic material comprising 45 to 55% by weight TiB2and 40.5 to 54.7% by weight BN and 0.1-1.5% by weight Ca in the form of a compound selected from the group consisting of the calcium oxides and calcium borates and mixtures thereof; which material contains from 0.2 to 3% by weight of a substance selected from the group consisting of
(1) a compound selected from the group consisting of the oxides, carbides and nitrides of the elements Si, Zr, and mixtures thereof; and
(2) a compound selected from the group consisting of the oxides, and carbides of the elements Al, Si, Zr, Ti and mixtures thereof.
The present invention further provides a process for producing a ceramic material,
said ceramic material containing 45 to 55% by weight TiB2 and 40.5 to 54.7% by weight BN and 0.1-1.5% by weight CaO; which material contains from 0.2 to 3% by weight of a substance selected from the group consisting of
(1) a compound selected from the group consisting of the oxides, carbides and nitrides of the elements Si, Zr, and mixtures thereof; and
(2) a compound selected from he group consisting of the oxides, and carbides of the elements Al, Si, Zr, Ti and mixtures thereof; comprising the steps of
introducing from 0.2 to 3% by weight of said of compound selected from the group consisting of compound (1) and compound (2), in finely distributed form into a TiB2/BN/CaO mixture to form a homogenous powder mixture for 2-component vaporizers; and hot pressing said homogenous powder mixture in a graphite mold using ram pressure, at tempertures of at least 1800xc2x0 C.